The present invention relates to an image processing device and an image processing method, and in particular, it can be suitably applied to a liquid crystal display device with backlight control.
As a power consumption reduction technology in a liquid crystal display device (LCD) with a backlight, peak ACL (Automatic Contrast Limit) control is known. In the peak ACL control, a peak value of luminance, that is, the highest luminance in a video signal is detected, the luminance of the backlight is reduced to a necessary minimum value for the display of the peak value concerned, and luminance modulation is performed to the whole video signal such that a video signal output of a pixel with the peak value concerned becomes 100%. For example, if the luminance of the brightest pixel, that is, the peak value within a frame is 50% of the maximum luminance of the display unit, the backlight luminance is reduced to 50% and the luminance modulation is performed so as to double the video signal of the frame. The luminance displayed by a 100% backlight luminance multiplied by a 50% video signal is the same as the luminance displayed by a 50% backlight luminance multiplied by a 100% video signal. Accordingly, it is possible to reduce the power consumption of the backlight without reducing the luminance of the picture displayed.
On the other hand, a technology to compensate a video signal in order to improve the luminance in terms of human visibility is known. To the ordinary video signal, correction to compensate a gamma characteristic of a display panel (gamma correction) is performed generally. In the ordinary gamma correction, the correction is made so that the video data and the display luminance may have a proportionality relation, by offsetting the gamma characteristic of the display panel. On the other hand, it is possible to improve the luminance in terms of human visibility, by adjusting the amount of correction of the gamma correction to the direction in which the relation of the display luminance to the video data is shifted to the high luminance side from the proportionality relation as a whole. It is also possible to improve the luminance in terms of human visibility, by performing the correction so that the relation of the display luminance to the video data becomes smaller in the low luminance side and larger in the high luminance side than the proportionality relation, thereby enhancing the contrast.
Patent Document 1 discloses a display unit which aims at the improvement in the visibility of a low luminance gradation in a backlight control system for a power saving LCD. The display unit includes an APL curve setting unit which adjusts the backlight luminance based on the average luminance (APL: Average Picture Level) of a video signal; a luminance histogram modulator which modulates the backlight luminance and a gradation signal based on a luminance histogram; and a black correction unit which performs gamma correction of the modulated gradation signal (K2) based on a gamma value set in advance (refer to FIG. 2 of Patent Document 1). The black correction unit selects a gamma value in a gamma information storage unit which stores the correspondence relation of plural gamma values and the combination of the backlight luminance (D2) and the luminance signal (F), adjusted by the histogram luminance modulator. Since the optimal gamma value is calculated according to the backlight control value and the environmental illumination, it is possible to improve the visibility in a low luminance gradation.